Raman spectroscopy, which involves an inelastic scattering of photons by chemical entities, has been widely used as a tool for the identification of various chemical substances such as diamond, drugs, and biomolecules, as well as for investigation into adsorbed molecules on surfaces. However, the detection sensitivity of Raman spectroscopy and therefore its applications are often limited by the weak signal(s) associated with the intrinsically small Raman scattering cross-sections.
Since 1974, the discovery of surface-enhanced Raman spectroscopy, capable of strengthening the Raman signal and facilitating the identification of vibrational signatures of molecules in chemical and biological systems, has drawn substantial attention in the relevant field. Recently, the introduction of single-molecule Raman scattering further enhanced the Raman detection sensitivity, thereby broadening the scope of sensor applications involving SERS.
A SERS-active substrate based on nanosphere lithography-derived Ag particles, which has adjustable surface plasmon resonance properties, has been demonstrated (Hynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 7426; Jensen, T. R.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 2394). Xu et al. provided a theoretical study in 2000 that the effective Raman cross section of a molecule placed between two metal nanoparticles could be enhanced by more than 12 orders of magnitude (Xu, H.; Aizpurua, J.; Käll, M.; Apell, P. Phys. Rev. E 2000, 62, 4318). The field enhancement for SERS from metal nanoparticle arrays has also been theoretically investigated. Specifically, it was proposed that very localized plasmon modes, created by strong electromagnetic coupling between two adjacent metallic objects, dominate the surface enhanced Raman scattering response in an array of nanostructures (García-Vidal, F. J.; Pendry, J. B. Phys. Rev. Lett. 1996, 77, 1163). The interparticle coupling-induced enhancement contributes to the broadening in the width of the plasmon resonance peak, which better encompasses both the excitation wavelength and Raman peak. From the calculations of the average enhancement factor over the surfaces of an array of infinite-long Ag nanorods with semicircular cross-section, it has been shown that significant near-field interaction between adjacent nanorods takes place when the gap between the nanorods reached half (½) of their diameters.
The dependence of the enhancement factor on the gap between the adjacent nanoparticles on a SERS-active substrate has also been studied. For example, Gunnarsson et al. reported SER scattering on ordered Ag-nanoparticle arrays with interparticle gap above 75 nm (Gunnarsson, L; Bjerneld, E. J.; Xu, H.; Petronis, S. Kasemo, B.; Käll, M. Appl. Phys. Lett. 2001, 78, 802). Lu et al. provided the study on the temperature-controlled variation of interparticle gaps among Ag-nanoparticles embedded in a polymer membrane (Lu, Y.; Liu, G. L.; Lee, L. P. Nano Lett. 2005, 5, 5). Performance of SERS on self-organized Au-nanoparticle arrays with narrow interpaticle gap was investigated, and so was SERS from nanowire arrays in aluminum matrix with interparticle gaps of ˜110 nm. (Wei, A.; Kim, B.; Sadtler, B.; Tripp, S. L. Chem. Phys. Chem. 2001, 2, 743; Sauer, G.; Brehm, G.; Schneider, S.; Graener, H.; Seifert, G.; Nielsch, K.; Choi, J.; Göring, P.; Gösele, U.; Miclea, P.; Wehrspohn, R. B. J. Appl. Phys. 2005, 97, 024308).
As shown by the theoretical and experimental studies above, the precise control of the gaps between the nanostructures on a SERS-active substrate to be in the sub-50 nm range is difficult with the pre-existing nanofabrication methods. It is thus the intent of this invention to control the inter-nanopit gaps to be around or below 50 nm, as that is the key to the fabrication of SERS substrates with uniformly high enhancement factor.